The inventions described herein relate generally to methods and installations for cutting glass pieces.
The methods and installations can be used both in the flat glass industry and in the hollow glass industry but will be described in more detail for their application to hollow glass.
The manufacture of objects in hollow glass such as drinking glasses, vases etc generally includes an operation known as “cracking-off”.
Cracking-off is an operation by means of which there is separated, from the glass part constituting the final glass piece proper, a mass of glass which does not constitute the final glass piece proper but which results from the hot forming process; this is in general the part by which there was held the mass of glass initially intended to constitute the piece of glass during the pressing or blowing operations which were performed when the glass was in the viscous state in order to give it the required shape corresponding to that of the glass piece. This part of the glass piece not constituting the final article proper is in general referred to as “moil”.
Cracking-off can be carried out by a hot method or a cold method.
During hot cracking-off, the glass is raised, in the vicinity of the area where the cracking-off is to occur, to a temperature greater than the softening point, where it becomes malleable: separation occurs by sufficiently moving apart the two parts of glass to be separated, when the two masses of glass separate, the surface tensions act in order to form a thick rounded edge. This method is in general reserved for lower-quality articles.
During cold cracking-off, the glass is raised, in the vicinity of the area where the cracking-off is to occur, to a temperature below that of the case of hot cracking-off; the glass remains in the fragile range and the purpose of this heating is merely to introduce mechanical tensions which, in being released, for example under the effect of contact with a cold metallic object or under the effect of a cold air jet, will cause a crack which, propagating in the fragile material, will cause cracking-off. In the case of cold cracking-off, it is frequent for the method used to include a marking stage whose purpose is to generate, on the surface of the glass article, through the mechanical action of a tool which is harder than glass, a surface defect which can be assimilated to a scratch, and which will serve to guide the propagation of the crack when it occurs abruptly. The result of this method is a thin but sharp edge which, in order to have the properties necessary to the finished glass piece, will have to undergo various polishing operations by means of abrasive tools, in general referred to as flatting, bevelling, flame polishing, and even toughening in order to give it sufficient mechanical strength. The final result is a thin and sufficiently strong edge. The cold cracking-off operation is more complex to perform than hot cracking-off and is reserved for higher-quality glass pieces.
The flatting operation is described notably in the document BE 670 504.
The optimum performance of the cold cracking-off operation makes it possible to obtain an edge as close as possible to its final form, having as few defects as possible so that the flatting operation is carried out easily, without requiring removal of a great deal of material.
The cold cracking-off operation by means of the conventional methods does not always give good results or gives results which are insufficiently reproducible on glass pieces having particular shapes, such as for example splayed glasses or glasses which are substantially cylindrical but have a polygonal cross-section. The mechanical appliances used for effecting marking by means of diamond tip, or a carbide or ceramic tip or wheel, must allow a multitude of adjustments in order to be able to adapt to the different geometries of the glasses to be processed, and in all cases they require regular maintenance and the intervention, at each change of production, of specialist staff.
For some time, methods resembling the cold cracking-off method, using one or more beams of laser rays for heating the material and generating therein the necessary defect equivalent to the grid of the tool, have been proposed.
Through the patent application WO98/56722, a method of cracking-off glass pieces is known, this method comprising two successive steps:    in a first step, a piece to be cracked-off under rotation is subjected to the action of a continuous beam of laser rays or to continuous beams of laser rays;    in a second step, for substantially one rotation of the piece to be cracked-off or for at least one rotation of the piece to be cracked-off, the area which was in the first step subjected to the action of one or more continuous beams of laser rays is subjected to the action of at least one pulsed beam of laser rays focused so as to form a series of successive points distant from each other, the series defining a substantially continuous line along which the cracking-off substantially takes place.
The result is a very even, almost plane cracked-off edge, except for irregularities of small magnitude, easily able to be eliminated by light flatting, inexpensive and easy to perform; the method makes it possible to obtain this even edge in a highly reproducible fashion. In addition, for certain glass pieces, in particular for pieces of simple shape, for example cylindrical or substantially cylindrical, and for glass pieces with a wall thickness generally less than 2 mm, this method makes it possible to obtain a cracked-off edge able to be processed directly in a flame-polishing station, without having to be previously flatted.
The main drawback of the method described in the patent application WO98/56722 is requiring the use of at least two distinct beams of laser rays, of different natures: a continuous beam of laser rays and a pulsed beam of laser rays, which complicates the installation implementing the method and increases its installation and operating cost.